This map is starting to show its age, it was made over 12 years ago, with what is now old technology; which alas many modern browsers no longer fully support.
In particular the "open in new window"/tab does NOT function propelly, nor normal left-clicking on the map at thumbnail scale for some squares. And using other than 100% page zoom can cause issues.
For best results use Internet Explorer 10 or below (yes really!), or Firefox still seems functional.